Biota of Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 171

mosquitoes, to a year or longer (Cushing and Allan 2001), while the adult may commonly live for
a month or less.
In incomplete metamorphosis (hemimetabolous development), there are typically three main
stages of development: the eggs, immature insects called nymphs that undergo a series of molts
until they become adults. There is no intermediate pupae stage where transformation occurs. The
nymphs typically resemble the adults except for their wing development. The damselly, mayly,
and dragonly are examples of insects that undergo incomplete metamorphosis. Maylies undergo
incomplete metamorphosis, where they may spend 11-24 months as a nymph, and undergo 20-30
molts. They then rise to the surface, take light, and undergo a inal molt to become an adult with no
functioning mouthparts, so they cannot eat; subsequently, they reproduce and die.
Some insects can have several generations per year (referred to as multivoltine), one generation
per year (univoltine), or one generation every 2-3 years (semivoltine). The cycle is based largely on
temperature, where a certain temperature history (number of days with a certain temperature) will
trigger the transformation of an adult. The temperature history is usually measured in degree-days, the
cumulative product of the temperature and the time between some development threshold. Maylies
typically require 2000+ degree-days before emerging (Cushing and Allan 2001). Insects are also vari-
ously adapted to live in rivers and streams. Some of the common adaptations are listed in Table 6.1.
Crustaceans, also in the phylum Arthropoda, include the orders Isopoda (aquatic sow bug),
Decapoda (crayish), and Amphipoda (scud/sideswimmers) (Figure 6.8). Crustaceans are found in a

TABLE 6.1
Insect Adaptations to Living in Streams and Rivers

Representative Groups
and Structures

Adaptation Signiicance Comments

Dorsoventrally lat Allows crawling in slow-current

boundary layer on substrate

Odonata—Gomphidae
Trichoptera—Glossosoma

Streamlining Fusiform body minimizes

resistance to current

Ephemeroptera— Baetis
Diptera—Simulium

Relatively rare body form

Reduced projecting
structures

Reduces resistance to current Ephemeroptera— Baetis Large lateral structures

exist in some groups

Suckers Attach to smooth surfaces Diptera—Blephariceridae Rare adaptation

Friction pads Increased contact reduces chances

of being dislodged

Coleoptera— Psephinus

Small size Allows use of slow-current

boundary layer on top of
substrate

Stream animals are smaller
than stillwater relatives

Silk and sticky
secretions

Attachment to stones in swift
current

Diptera— Simulium
Trichoptera—Hydropsychidae

Ballast Cases made of large stones Trichoptera— Goera

Attachment claws/
dorsal processes

Stout claws aid in attachment to
plants

Ephemeroptera— Ephemerella

Reduced power of
light

Prevents emigration from small
habitats

Plecoptera— Allocapnia Reduces dispersal ability

Hairy bodies Keeps sand/soil particles away

while burrowing

Ephemeroptera— Hexagenia Allows water low over

body

Hooks or grapples Attachment to rough areas of

substrates

Coleoptera—Elmidae

Source: Developed by Merrick, R.; Water on the Web, Monitoring Minnesota Lakes on the Internet and training water sci-
ence technicians for the future—A national on-line curriculum using advanced technologies and real-time data,
University of Minnesota, Duluth, MN, 2004. Available at http://WaterOntheWeb.org.